Final Kill

Final Kill is unlocked once Loose Ends and Hotshot Lead 4 are completed. Talk to Mrs. Chu, then head to Dogeyes' (Sammy's) hideout in North Point.

Enter the compound on foot and chase him on boat. Run after him in the building, fight his thugs and take Dogeyes back to the restaurant. Protect the car while you go back, which should be easy with infinite ammo.